Estimation of material properties can be used in material classification, discrimination and identification. Optical methods of estimation generally have the benefit of not requiring physical contact with a material sample and avoiding potential damage to the sample. Examples of material properties that can be estimated by optical measurement of scattering of light include surface roughness, spectral reflectance, bidirectional reflectance distribution function (BRDF), bidirectional surface scattering reflectance distribution function (BSSRDF) and the like. The scattering of light can be due to surface microstructure (in the case of surface roughness) or sub-surface microstructure (in the case of sub-surface scattering described by the BSSRDF).
In the field of measuring surface roughness of a material, it is known that there is a correlation between surface roughness and intensity statistics of a speckle pattern. A speckle pattern typically results from interference of light waves scattered by a material when illuminated by a controlled coherent light source such as a laser. The nature of the speckle pattern depends in part on surface roughness, and it is often possible to estimate surface roughness by deriving a speckle statistic from the speckle pattern.